Problem: In physics class, the girl to boy ratio is $9$ to $5$. If there are a total of $56$ students, how many girls are there?
Answer: A ratio of $9$ girls to $5$ boys means that a set of $14$ students will have $9$ girls and $5$ boys. A class of $56$ students has $4$ sets of $14$ students. Because we know that there are $9$ girls in each set of $14$ students, the class must have $4$ groups of $9$ girls each. There is a total of $36$ girls in physics class.